The present invention relates to skewing compensation in a laser based image forming system, and, more particularly, to a technique for deskewing within a laser-based image forming system comprising a plurality of laser beams.
The formation and development of latent images on the surface of photoconductive materials using liquid or powder developing material is well known. The basic process involves placing a uniform electrostatic charge on photosensitive plate and exposing the layer to a light or to a scanning laser to dissipate the charge on the areas of the plate exposed to the light to form a latent electrostatic image.
An example of the known art is shown in FIG. 1. Generally speaking known art laser based printing system 10 uses a laser source 12 that produces a single beam 13 that is scanned optically over a photosensitive plate 14 using an optical element such as polygon 16. Incidence of the laser beam 13 on the plate discharges the plate 14 at points struck by the beam and a latent image is formed on the plate in accordance with the scanning of the beam. The result is an electrostatic image on the photosensitive plate 14.
In addition to the above, systems are known in which multiple laser beams are scanned simultaneously over the plate.
The resultant latent image is developed by subjecting the latent image to a liquid toner (in case of liquid ink development) comprising a carrier liquid and colored toner particles. Generally , the development is carried out in the presence of an electric field, such that the charged toner particles are attracted either to the charged or discharged areas, depending on the charge on the particles and the direction or the magnitude of the field.
This image is transferred by means of electrical field to an intermediate transfer member (ITM) 20 which is typically covered with a replaceable blanket 22. The blanket is kept at elevated temperature and the carrier liquid is evaporated. The resultant tacky ink film is transferred from the blanket, by thermal forced to a sheet 26 which is located onto impression drum 24. The transferred image is permanently affixed to the substrate by application of heat and pressure.
It should be mentioned that there are also systems which do not use a blanket. Instead they transfer the image directly from the photosensitive plate to the substrate.
A disadvantage of the device of FIG. 1 is that the single beam is required to scan and form the entire image, thereby acting as a limitation on the printing speed. In order to overcome this problem a multi-beam laser source was developed and the writing speed was thereby increased. Such a laser source is illustrated in FIG. 2. A single integrated circuit or chip or other aggregate of lasers 30 provides a multi-beam laser source, having a plurality of individual laser beam sources 32 which scan the image in parallel through polygon 16, forming simultaneously a group of scan lines. Each scan line corresponds to a respective individual laser beam. The laser source may be a diode laser array with individually addressable laser diodes. Such an array may be provided on a single integrated circuit.
Now, in modern high definition printing a typical spot or pixel size is approximately 31 microns, equivalent to a scan time of around 13 nanoseconds, although typically a range of 21 to 42 microns is found, and the scan times have a corresponding variation range. Even if it is physically possible to build the multi-beam laser source such that the individual laser sources are at the corresponding spacing, in our example at a 31/M micron interval or spacing,(M=3÷7 being a typical system optical magnification) but current manufacturing techniques mean that at such a scale the variation in the spacings between the individual sources is likely to be a large and noticeable percentage of the spacings themselves. It is therefore conventional to build a laser source with the spacings much larger than the required 31/M microns so as a result the spacing variations are a relatively small percentage error and then skew the source in the scan direction as shown in FIG. 3 to give the correct inter beam spacing. Typically this spacing is 100 micron. That is to say multi-beam source 40 is skewed against the direction of scan indicated by arrow 42 so that the skew angle effectively cancels out the manufacturing error in the spacings between the laser sources.
Now it will be appreciated that printing a straight vertical line, vertical meaning orthogonal to the scanning direction, using the skewed source in FIG. 3 requires that the successive laser beams are switched with successively increasing delays, since each successive beam reaches the same position in the scan later due to the skew. The process of altering the timing of the beams to compensate for the skew is known as deskewing.
In order to perform deskewing there are numerous factors that need to be taken into account. The delay needed by the different beams due to the physical spacing in the horizontal direction multiplied by optical system magnification, is one issue. Secondly there are electrical switching delays in the circuitry, specifically electronic delay in the driver board, which mean that there is a finite delay between the instant a particular beam is switched electronically and the moment the optical beam is produced. The delay is variable depending on the specific board and is due to factors such as parasitic capacitance that can vary between boards.
The extent to which each factor needs to be taken into account is the extent of the writing resolution accuracy, which is to say that typically the total spot size is equivalent to a scanning time of around 10-20 nanoseconds. In order to achieve such a resolution, issues of an order of magnitude below this should be considered. Such issues should therefore address positioning accuracy equating to a scan time of 1ns or better.
To date there are two methods being used for carrying out deskewing. First of all there is what may be termed “theoretical calculation” and is also referred to as an “open-loop” process. Theoretical calculation means simply building in the delay electronically in advance based on knowledge of the speed of rotation, the spacing between adjacent beams and the optical magnification. However theoretical calculation fails to deal adequately with optical magnification and also fails to deal at all with delays in the driver board since the driver boards are not sufficiently uniform. Two different driver boards can easily give very different delays, depending on parasitic capacitance and other effects as explained above. Thus the use of “theoretical calculation” results in print location errors that result in visual artifacts in the printed image. In particular in color images the visual artifacts come into existence due to interaction between grey scales between different screens.
A second method used to date for calibrating the deskewing involves carrying out a test operation in an optical laboratory, in which an attempt is made to print out an accurate straight line. The method takes advantage of the fact that straightness of a line is relatively easy to determine, simply by measuring using optical equipment. The resulting beam positions are measured in the focal plane of the system, and compensatory delays are applied until the line actually is straight. The Writing Head is then approved by the optical laboratory.
The optical laboratory method is relatively accurate, and it successfully takes into account all of the component parts of the delay in the beam, whether directly connected to the skew or otherwise, however it also has a number of disadvantages. First of all it is slow and costly to have to send each writing head to an optical laboratory and carry out accurate measurements if accurate calibration is needed. Secondly driver boards may be changed several times during the life of the writing head and it is not practical to recall the writing head for calibration every time a driver board is changed.
A third disadvantage of both methods is that parameters of the printer, including delays in the driver board, tend to drift over the lifetime of the printer. Thus even the most accurate determination of the deskew parameters in the laboratory prior to sale cannot be guaranteed to prevent the gradual appearance of artifacts in the printout over the life of the printer.
There is thus a widely recognized need for, and it would be highly advantageous to have, a skew compensation, or deskewing, system devoid of the above limitations.